Primordial Nucleosynthesis Redux Walker, Steigman, Schramm, Olive,

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Primordial Nucleosynthesis ReduxWalker,
Steigman, Schramm,Olive, Kang (1991)

• Ast 541 Seminar
• Nicole Lemaster
• Nov 17, 2004

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Introduction

• Tests of standard big bang cosmological model
• Primordial nucleosynthesis synthesis of light
  elements from t 1 to 1000 s, T 0.1 MeV
• Cosmic Background Radiation t 105 yrs, T 1
  eV
• Cosmological models can be tested by comparing
  predicted light element abundances with observed
  abundances
• BBN can be used to constrain
• Cosmological parameters (e.g. ?b)
• Elementary particle physics parameters (e.g. N?)

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Standard Big Bang Model

• Has fewest adjustable parameters, based on
  observed large-scale isotropy and homogeneity of
  universe
• Assuming N? from known number of neutrino species
  and the validity of GR, abundances from BBN
  depend only on ?nb/n?
• n? is known from CBR measurements
• Therefore can determine baryon density from
  primordial light element abundances

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Outline

• Introduction
• Historical approach
• Physics of nucleosynthesis
• Abundance determinations
• Cosmological results
• Conclusions
• Recent developments

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Historical Approach

• BBN calculation method hadnt changed since 1950
  when Hayashi recognized the importance of the n/p
  ratio
• Discovery of CBR in 1960s didnt change method
• Focus for 25 yrs prior to this paper was on
  understanding how light element abundances could
  tell us about the early universe
• In 1960s, focus was on 4He
• Very insensitive to ? ? provided support for
  model but no predictions

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T Tauri Light Element Synthesis

• First proposed by Fowler, Burbidge, and Burbidge
  (1955) and Hayakawa (1955)
• T Tauri stars
• Phase between protostar and main sequence stages
• Do not yet burn nuclear fuel
• Only available energy comes from gravitational
  contraction
• Thought that a period of intense electromagnetic
  surface activity could lead to light element
  production
• Ryter et al. (1970)
• If avg T few keV, energy necessary to produce
  light elements is greater than that available
  from gravitational contraction
• If hotter, stars in this phase would be
  observable in X-rays

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Historical Approach (contd)

• BBN became powerful cosmological tool
• Reeves et al. (1973) Cosmological origin of D
  and (at least some) 7Li universe cannot be
  closed by baryons alone
• Epstein, Lattimer, and Schramm (1976) no
  realistic astrophysical process could produce
  cosmologically significant amounts of D

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Purpose of Paper

• Use most recent reaction rates and observational
  abundance determinations to re-evaluate prior
  conclusions
• Concentrates mostly on BBN within standard model,
  but also tries to constrain other scenarios
• Uncertainties in rates and abundances now small
  enough that weak interactions must be followed
  more carefully

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Physics of Nucleosynthesis
A-B fusion rate per unit volume
S(E) is related to actual data by a Taylor
series expansion about E0
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Standard BBN

• Two stages of BBN
• Competition between expansion rate of universe
  and rate of weak interactions responsible for
  interconversion of n and p
• Competition between expansion rate and nuclear
  reactions that synthesize complex nuclei
• Primordial yields of D, 3He, and 7Li most
  sensitive to competition between nuclear reaction
  rates and expansion rate

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Standard BBN (contd)

• T 1 MeV n?p rates are greater than expansion
  rate of universe at equilibrium
• T 1 MeV n?p rates are less than expansion
  rate and n/p freezes out, except for neutron
  decay and collisions with e, ?e, anti-?e until
  nucleosynthesis begins
• T 100 keV equilibrium abundance of D becomes
  significant nearly all available neutrons are
  processed into 4He
• Absence of stable A5 or A8 nuclei, low density
  of nucleons, and n/p lt 1 make synthesizing nuclei
  beyond mass 5 difficult

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Standard BBN (contd)

• Calculated abundances of primordial 4He is most
  sensitive to n/p ratio at onset of
  nucleosynthesis
• Weak n?p rates can be expressed in terms of
  neutron decay rate
• Increasing nucleon number density
• Increases temperature at which D has an
  appreciable equilibrium abundance
• Increases efficiency of forming 4He
• ? Decreases primordial abundances of D and 3He
  while increasing that of 4He

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Approach to Problem

• Can test standard model and constrain
  non-standard models using abundances of light
  elements derived from observational data
• Testing theoretical predictions requires
  primordial abundances to be extracted from
  observations
• Observational uncertainties compounded by
  uncertainties in galactic chemical evolution
• Authors attempt to infer 95 CL (2?) bounds to
  primordial abundances of D, 3He, 4He, and 7Li
  from best data at time
• Authors rely on model independent bounds when
  possible, even though more stringent constraints
  can be made using specific models

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D and 3He

• D cycled through stars completely burned away
• Difficult to produce D in significant quantities
• Primordial D abundance no smaller than present or
  presolar abundance
• Presolar D destroyed during pre-MS evolution
• Could infer presolar D abundance from present 3He
  abundance
• But not all 3He comes from D

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D and 3He (contd)

• Smallest present 3He abundance
• Carbonaceous chondrites give presolar 3He/4He
• Can infer primordial 3He abundance
• Larger present 3He abundance
• Gas-rich meteorites, lunar soil, solar wind give
  present 3He/4He
• Can infer primordial abundance of D 3He
• Lower limit on primordial D abundance from
  difference in primordial 3He abundance and D
  3He abundance
• Primordial D must be in ISM or been destroyed in
  stars
• Upper limit on primordial D abundance by
  considering destruction of D and 3He in stars

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7Li Abundance

• Cosmological significance of 7Li increased with
  discovery of lithium in halo (Population II)
  stars
• Population II stars
• Metal poor
• Thought to have formed from primordial abundances
• Main sequence models of most metal poor
  Population II stars indicate essentially no 7Li
  depletion for hotter stars
• Primordial abundance of 7Li should be similar to
  present abundance in these stars

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7Li (contd)
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4He Abundance

• Easiest to observe in the universe (young stars,
  old stars, planetary nebulae, galactic and
  extragalactic HII regions)
• Primordial abundance contaminated by galactic
  evolution (stars burning H ? He)
• Least contamination in metal-poor environments
  (extragalactic HII regions)
• He observed in HII regions by recombination
  radiation ? neutral He is invisible
• Must use theoretical model of HII regions to
  estimate HeI contribution or restrict attention
  to hotter, higher excitation HII regions and
  neglect the HeI contribution

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4He (contd)

• Must still correct for 4He evolution in most
  metal-poor extragalactic HII regions
• Can infer 4He primordial mass fraction from
  present abundances of 4He, C, N, and O
• O Yp 0.229 0.004
• N Yp 0.231 0.003
• C Yp 0.230 0.007
• All consistent with Yp 0.23 0.01
• Difficult to determine true uncertainty in
  estimate due to systematic effects
• Ionization corrections
• Collisional excitation corrections
• Nonlinear detectors

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Standard Model

• Standard model N? 3, 882 ?n 896 seconds
• Use abundances to get range of nucleon-to-photon
  ratio
• D ?10 6.8
• D 3He ?10 2.8
• 7Li 1.6 ?10 4.0
• All consistent with 2.8 ?10 4.0
• Use nucleon-to-photon ratio range to get 0.236
  YpBBN 0.243 (agrees with Ypobs 0.23 0.01)
• Could use 4He data to derive bound on ratio, but
  would require highly accurate upper bound to 4He
  abundance

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Standard Model (contd)

• Successfully accounts for observed abundances
  of all the light elements

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Cosmological Results

• Nucleon-to-photon ratio is virtually unaltered
  from epoch of primordial nucleosynthesis to
  present
• Bounds on ? give bounds on current density of
  baryons
• Can find ?B ?B/?C using
• baryon mass density ?B MNnB
• critical mass density ?C 3H02/8?G
• Using present values
• Hubble parameter H0 50 h50 km/s/Mpc
• CBR temperature T 2.75 T2.75 K

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Results (contd)

• Using 95 CL range 2.75 K T 2.79 K
• Plugging in allowed range of nucleon densities
• Hubble parameter not well known at time
• Could be as low as H0 40 km/s/Mpc ? ?B 0.10
• Could be as high as H0 100 km/s/Mpc ? ?B 0.01
• Corresponds to young universe t0 H0-1 9.8
  Gyr
• Using ?TOT 1 ? t0 (2/3)H0-1 13h50-1 Gyr ?
  ?B 0.04 for t0 13 Gyr

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Results (contd)

• Universe fails to be closed by baryons by at
  least factor of 10
• If ?TOT 1, nonbaryonic matter required
• Luminous matter in galaxies ? ?LUM 0.007
• Significant fraction of nucleons in universe are
  dark ?B/?LUM 6h50-2
• Nonbaryonic dark matter highly suggested but not
  required given ?TOT 0.2 0.1 from rich
  clusters and large-scale flows

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Results (contd)

• Comparison between theory and observation ? Yp
  0.240, N? 3.3
• BBN bounds to N? are sensitive to precise value
  of upper bound to Yp

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Suggestions

• The current successes of standard BBN should not
  be a cause for complacency.
• More observations of 7Li in Pop II stars
• More modeling of evolution of Pop II stars
• Better models of chemical evolution of galaxy ?
  better bounds on pre-galactic abundances of D and
  3He

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Conclusions

• Improvements in primordial abundance estimates ?
  ongoing checks of consistency of standard model
• If standard model is correct, can zero in on
  actual value of ?
• Detailed studies of selected extragalactic HII
  regions helpful in determining systematic errors
• Upper bound on Yp to third significant figure ?
  could constrain N? lt 3 ? key test of standard
  model

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Citations to Article

• Cited 781 times since published in 1991
• Cited 96 times by authors of this paper!
• Topics of citing papers
• Cosmic Microwave Background
• Dark matter
• Big bang nucleosynthesis
• Non-standard cosmologies
• Neutrinos
• Gravitational waves

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Recent Developments

• Still no major changes in calculation method
• Olive, Steigman, and Walker (2000)
• Possibility of systematic errors in derived
  primordial abundances much larger than
  statistical errors ? wide bounds on abundances
• Using H0 70 km/s/Mpc and 0.228 Yp 0.248

High-D, low-? range 1.2 ?10 2.8 0.01
?B 0.02 N? 4.8
Low-D, high-? range 4.2 ?10 6.3 0.03
?B 0.05 N? 3.3
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Summary

• Universe not closed by baryons alone
• Given density estimates of luminous matter,
  significant fraction of nucleons in universe are
  dark
• 4He abundance bounds ? N? 3.3
• If observed value of Yp reduced below 0.236 ? N?
  lt 3 ? BBN is falsifiable theory